Hybrid electric powertrains typically have engine autostop functionality in which a controller shuts off a fuel feed to an internal combustion engine during idle conditions in order to maximize fuel economy. A belted alternator starter (BAS) system may be used in conjunction with or in lieu of an auxiliary starter motor to quickly restart the engine after the autostop event, or to assist the engine during specific operating conditions to increase torque into or from a transmission. In a BAS system, a high-voltage motor-generator unit (MGU) is operatively connected to a crankshaft of the engine via pulleys and a chain or belt. Motor torque from the MGU cranks and starts the engine during an engine autostart event. Motor torque from the MGU in some BAS systems may also be used to selectively assist output torque from the engine in an electrical assist mode.
The MGU of a BAS system suitable for a wide range of engines is typically rated for voltages higher than 12-15 VDC auxiliary levels so as to minimize operating current. A higher-voltage battery is thus used for powering the MGU and other higher-voltage electrical devices such as positive temperature coefficient (PTC) heaters, coolant pumps, active chassis control devices, and the like. A separate lower-voltage auxiliary battery is used to run various motor vehicle accessories such as headlights, heating, ventilation, and air conditioning (HVAC) devices, auxiliary motors, and entertainment system components. The auxiliary battery may be recharged in some configurations via a DC-DC converter, a device which reduces an output voltage of the higher-voltage battery to suitable auxiliary voltage levels.